本節(jié)課分成五部分：

1. Obligation - present
2. Obligation - past
3. Probability - present
4. Probability - past
5. should

1. Obligation - present

rules/regulations

You must check in 40 mins before the time.

forbiden/prohibited - must

have to: necesary

You have to practice if you want to improve

I have to .....

have to (in negative)

-------------------

you must not : it's forbidden

you don't have to: you can if you want

2.past

We had to check in 40 mins before ..

3.probability

He must be working

I must have the wrong number.

It has to be the right number. (no other explanation)

It must be Andrew.

It has to be Andrew.

4.Probability - past

must have been ..

I must not have had the wrong number.

I must have been his brother.

It had to have been his brother.

5.should

what option is correct/the best/my recommendation?

should + have + pp

His parents didn't go to his birthday party.

They should have gone to his party.